Why does my QCX+ #40m still work? #40m


I recently completed my QCX+ 40m (excellent build instructions Hans) and apart from a presumed dry solder joint that resulted in no audio and a loud click when keying... which is now fixed... my QCX appears to be working very well.

However, while trouble-shooting the above problem I measured the voltages as described on p.171 & 172 of the build manual and got some very unexpected results.

First up is IC5.
Pin 3 should be 2.26V, I measure 1.69V
Pin 5 should be 2.28V, I measure 1.61V

Next is the LCD.
Pin 4 should be 4.92V, I measure 0.04V!!!
Pin 13 should 4.92V, I measure 0V!!

I've traced back LCD pins 4 and 13 to IC2 pins 27 and 4 respectively. There are no shorts or breaks anywhere along the path that I can see. There is 0.04V on pin 27 of IC2 and 0V on pin 4 of IC2, yet both should be 4.92V.

For the most part all other voltages are within tolerance, with a few exceptions...

T1 pins 5,6,7,8 should be 2.36V, I measure 1.79V
Top side of C43 should be 2.28V, I measure 1.71V 
Top side of C44 should be 2.44V, I measure 2.13V
Top side of C45 should be 2.42V, I measure 2.20V
Top side of C46 should be 2.30V, I measure 1.63V 

But my primary concern is the 0V on pins 4 and 27 of IC2. What is the implication of that because my QCX+ appears to be fine??!?

Aside: The DC voltage table mentions to measure the left side of C2 and the right side of D2. That worked for the QCX but is impossible of the QCX+ because those components are perpendicular! I believe it should say to measure the bottom of D2??

Thanks in advance for your suggestions.

73 Ian


What did you use to measure the voltages?




I used the internal QCX+ DVM initially, as that is what Hans used for the documented voltages, but I then confirmed them with an external DVM. 

73 Ian



Continue to check things out, but if all is well with the qcx its a quirk indeed. Yes painful to have the nice test equipment in the qcx that you didn't need (-:

Actually I did need the built in RF source to find my own missed solder connection on my first qcx. Don't let the puzzle get to you if the rig is working well.

73 curt


On Thu, Sep 24, 2020 at 08:21 PM, Ian VA7ITM wrote:
But my primary concern is the 0V on pins 4 and 27 of IC2. What is the implication of that because my QCX+ appears to be fine??!?(
Ian, looking at the schematic, pin 4 on the lcd is reset (active hi), so should be 0v except maybe a short 5v pulse at power up. Pin 13 is an lcd data line so you may only see short 5v pulses with a ‘scope, this may look like 0v using a dvm.
if it ain’t broke, don’t fix it.

73 Ken G4APB


Thank you all for your replies and sorry for the tardy response.

Alas, I should have known titling my thread "Why does my QCX+ still work?" was tempting fate.

The problem I mentioned in my post about no audio and loud clicks when keying... which I thought I had solved... has returned again :-(

I will post about this as a new thread but before I do I will do some homework first, recheck all the solder joints and work my way through the trouble-shooting manual.

73 Ian